An anti-reflection film proposed in the art is of a multi-lamination structure where seven component layers are deposited one over another on a substrate ranging from 1.4 to 1.9 in refractive index at an arbitrary design basis wavelength within a wavelength range near visible band (see Patent Document 1 listed below).
Such a film configuration, since it has no thin film, hardly causes optical unevenness and tolerance of manufactured film thickness, and resultantly brings about an enhanced anti-reflection effect throughout the visible light band.
Some of other anti-reflection films in the prior art have a multi-lamination structure where a substrate of high refractive index equal to or greater than 2.0 underlies six component layers deposited one over another, alternating the one of high refractive index and the other of low refractive index. The closest, and third and fifth closest component layers to the underlying substrate are of film of “high refractive index” although lower in refractive index than the underlying substrate, the second and fourth closest component layers to the underlying substrate are of film of “middle or low refractive index”, and the sixth closest or uppermost component layer is of film of “low refractive index” (e.g., see Patent Document 2 listed below). The anti-reflection film can successfully reduce a reflectivity down to 0.5% or even lower at a wavelength ranging from 400 nm to 700 nm, namely, throughout the entire visible band.
Still another prior art anti-reflection film is of three-lamination dielectric film structure where first, second and third component films, which are queued down as denoted by ordinal numbers from a light entrance medium primarily of air toward the surface of an optical component body, have their respective refractive indices n1, n2 and n3 correlated as can be expressed in the following formula:n1<n2<n3 and have their respective film thicknesses (spectrometric wavelengths) n1d1, n2d2 and n3d3 correlated in terms of the design basis wavelength (e.g., see Patent Document 3 listed below). Simply with such a three-lamination thin film structure stacked over the optical component body, the anti-reflection film can reduce a surface reflectivity in a wide visible light band, and since its processing method is not complicated, the manufacturing cost can also be reduced.
Optical devices having any of the prior art anti-reflection films include the one disclosed as having an anti-reflection film that consists of an intermediate refractive index film layer, a high refractive index film layer overlying the intermediate refractive index film layer, and a low refractive index film layer further overlying the high refractive index layer where the intermediate refractive index film layer is made of a mixture of substances of the high and low refractive index film layers (e.g., in Patent Document 4 listed below). This is conspicuous in that a reflectivity within the visible light band and a peak value of the reflectivity at the design center wavelength keep small independent of the refractive index of the substrate.
<Patent Document 1>                Claims and Detailed Descriptions in the Official Gazette of Japanese Preliminary Patent Publication No. H10-20102        
<Patent Document 2>                Claims and Detailed Descriptions in the Official Gazette of Japanese Preliminary Patent Publication No. 2000-347002        
<Patent Document 3>                Claims and Detailed Descriptions in the Official Gazette of Japanese Preliminary Patent Publication No. H4-260001        
<Patent Document 4>                Claims and Detailed Descriptions in the Official Gazette of Japanese Preliminary Patent Publication No. 2003-202405        
The anti-reflection film disclosed in Patent Document 1 is of a multi-lamination structure consisting of seven layers over a substrate ranging from 1.4 to 1.9 in refractive index at an arbitrary design basis wavelength within a wavelength range near visible band, and if the substrate is 2.0 or higher in refractive index, the reflectivity is prone to rise up to 15 to 20%.
The anti-reflection film disclosed in Patent Document 2 is deposited over a substrate of a high refractive index of 2.0 or even greater, and the film is of a multi-lamination structure consisting of layers as many as six in number, which results in an increased manufacturing cost.
The anti-reflection film of the three-lamination dielectric film disclosed in Patent Document 3 is, simply with three thin films deposited over the optical component body, capable of reducing a surface reflectivity within a wide band of visible light, and since its processing method is not complicated, the manufacturing cost can also be reduced. Under such requirements of the refractive index and the film thickness, however, the anti-reflection film cannot retain its reflectivity 1% or under within a visible band from 400 nm to 700 nm when deposited over the substrate of 1.90 or greater in refractive index, which attains an insufficient anti-reflection effect within the visible band.
The anti-reflection film for optical devices as disclosed in Patent Document 4 is of a three-lamination structure consisting of the intermediate refractive index layer, the high refractive index layer overlying the intermediate refractive index layer, and the low refractive index layer further overlying the high refractive index layer. Under the requirements of component film thickness (i.e., spectrometric wavelength) for the queued arrangement of the substrate—0.25λ thin film—0.5λ thin film—0.25λ thin film—the air, however, the anti-reflection film applied over the substrate of 1.90 or greater in refractive index cannot retain the reflectivity 1% or below within the visible band from 400 nm to 700 nm, which attains an insufficient anti-reflection effect within the visible band.
The present invention is made in order to overcome the aforementioned disadvantages of the prior art anti-reflection films, and accordingly, it is an object of the present invention is to provide an anti-reflection film of the increased anti-reflection performance that can be applied to an optical material of high refractive index for the purpose of downsizing optical components.
It is another object of the present invention to provide a cost reduced anti-reflection film of a multi-lamination structure that is reduced in number of layers and increased in thickness of the same for the purpose of facilitating the total control of the film thickness and that can be manufactured in a simplified process so as to reduce the manufacturing cost.